Extending the WP-DarkSky helper class and adding some Weather Icons by Eric Flowers.

wp-darksky-weather-icon-example.php

<?php
/**
 * A helper class for that extends the WP-DarkSky helper class with Weather Icons.
 * 
 * You'll need to add the Weather Icons stylesheet, web fonts, etc and enqueue them correctly.
 *
 * @see weathericons.io
 * @see https://developer.wordpress.org/reference/functions/wp_enqueue_style/
 * @see https://darksky.net/dev/docs/
 *
 * @link https://github.com/joshuadavidnelson/wp-darksky
 *
 * @since 1.0.0
 * @since 1.1.0 - Update for new Dark Sky API (versus old Forecast.io naming)
 *
 * @author Joshua David Nelson, josh@joshuadnelson.com
 */

$args = array(
	'api_key' 	=> $api_key, // set your api key
	'latitude'	=> $lat, // set your latitude
	'longitude'	=> $long, // set your longitude
	'query'		=> array( 'units' => 'us', 'exclude' => 'flags' )
);
$forecast = new DarkSky\Weather_Icon_Forecast( $args );

// Get the current forecast for a given latitude and longitude
$daily = isset( $forecast->daily['data'] ) ? $forecast->daily['data'] : '';

$current = '';
$title_attr = '';

if( is_array( $daily ) ) {
	$date_format = 'n/j/Y';
	$time_now = date( $date_format, current_time( 'timestamp' ) );
	foreach( $daily as $day ) {
		if( isset( $day['time'] ) && $time_now == date( $date_format, $day['time'] ) ) {
			// check the variables are setup
			if( isset( $day['temperatureMin'], $day['temperatureMax'] ) && !empty( $day['temperatureMin'] ) && !empty( $day['temperatureMax'] ) ) {
				
				// The temperature output
				$current = number_format( $day['temperatureMin'], 0 ) . ' / ' . number_format( $day['temperatureMax'], 0 );
				
				// Set the icon, if there is one
				$icon = !empty( $day['icon'] ) ? $forecast->get_icon( esc_attr( $day['icon'] ) ) : '';
				
				// set the title attribute to the forecast summary
				$title_attr = !empty( $day['summary'] ) ? ' title="' . esc_attr( $day['summary'] ) . '"' : '';
			}
		}
	}
}

// If we have a valid forecast, we can build the widget
if( ! empty( $current ) )
	echo '<div class="widget weather-widget"><h5 class="title" id="weather-widget-title">Today\'s Weather</h5>' . $icon . '<span class="temp">' . $current . ' &deg;</span></div>';

wp-darksky-weather-icon-forecast.php

<?php
/**
 * A helper class for that extends the WP-DarkSky helper class with Weather Icons.
 * 
 * You'll need to add the Weather Icons stylesheet, web fonts, etc and enqueue them correctly.
 *
 * @see weathericons.io
 * @see https://developer.wordpress.org/reference/functions/wp_enqueue_style/
 * @see https://darksky.net/dev/docs/
 *
 * @link https://github.com/joshuadavidnelson/wp-darksky
 *
 * @since 1.0.0
 * @since 1.1.0 - Update for new Dark Sky API (versus old Forecast.io naming)
 *
 * @author Joshua David Nelson, josh@joshuadnelson.com
 */

namespace DarkSky;

class Weather_Icon_Forecast extends Forecast {
	
	/**
	 * Weather Icons mapped for Forecast.io.
	 *
	 * @since 1.0.0
	 *
	 * @var string
	 */
	private $icons = array(
		'clear-day' 		=> 'wi-forecast-io-clear-day',
		'clear-night' 		=> 'wi-forecast-io-clear-night',
		'rain'			=> 'wi-forecast-io-rain',
		'snow'			=> 'wi-forecast-io-snow',
		'sleet'			=> 'wi-forecast-io-sleet',
		'strong-wind'		=> 'wi-forecast-io-wind',
		'fog'			=> 'wi-forecast-io-fog',
		'cloudy'		=> 'wi-forecast-io-cloudy',
		'partly-cloudy-day'	=> 'wi-forecast-io-partly-cloudy-day',
		'partly-cloudy-night'	=> 'wi-forecast-io-partly-cloudy-night',
		'hail'			=> 'wi-forecast-io-hail',
		'thunderstorm'		=> 'wi-forecast-io-thunderstorm',
		'tornado'		=> 'wi-forecast-io-tornado',
	);
	
	/**
	 * Build it.
	 * 
	 * @uses wp_array_slice_assoc()
	 * @uses wp_parse_args()
	 * 
	 * @since 1.0.0
	 * 
	 * @param array $args The array of arguments.
	 */
	public function __construct( $args = array() ) {
		parent::__construct( $args );
	}
	
	/**
	 * Grab a weather icon.
	 * 
	 * @since 1.0.0
	 * 
	 * @param string $icon_name The Dark Sky icon name.
	 * 
	 * @return string $icon Icon print out or empty string. 
	 */
	public function get_icon( $icon_name ) {
		if( array_key_exists( esc_attr( $icon_name ), $this->icons ) ) {
			return '<i class="wi ' . $this->icons[ esc_attr( $icon_name ) ] . '"></i>';
		} else {
			return '';
		}
	}
}